A zeolite is a crystalline alumino-silicate catalyst that is well known for its utility in several applications. Zeolites have been used in dealkylation, transalkylation, isomerization, cracking, and disproportionation processes, among others. Its well-ordered structure is composed of tetrahedral AlO4−4 and SiO4−4 molecules bound by oxygen atoms that form a system of pores typically on the order of 3 Å to 10 Å in diameter. These pores create a high internal surface area and allow the zeolite to selectively adsorb certain molecules while excluding others, based on the shape and size of the molecules. Thus, a zeolite can be categorized as a molecular sieve. A zeolite can also be termed a “shape selective catalyst.” The small pores of the zeolite can restrict reactions to certain transition states or certain products, preventing shapes that do not fit the contours or dimensions of the pores.
The pores of a zeolite are generally occupied by water molecules and cations. Cations balance out the negative charge caused by trivalent aluminum cations which are coordinated tetrahedrally by oxygen anions. A zeolite can exchange its native cations for other cations; one example is the exchange of sodium ions for ammonium ions. In some ion-exchanged forms, such as the hydrogen form of a zeolite, the catalyst is strongly acidic. The acidic active sites are useful for alkylation as well as many other reactions. For instance, zeolites can serve as a solid acid catalyst for Friedel-Crafts alkylations, replacing traditional aluminum trichloride and other liquid acid catalysts that can be corrosive and damaging to the reactor.
One alkylation reaction for which zeolite can be used as a catalyst is the alkylation of toluene with methanol and/or formaldehyde to form styrene. Styrene, also known as vinyl benzene, is an organic compound having the chemical formula C6H5CHCH2. The monomer styrene may be polymerized to form the polymer polystyrene. Polystyrene is a plastic that can form many useful products, including molded products and foamed products, all of which increase the need for production of styrene.
In the production of styrene, zeolite catalysts may be utilized. The zeolite used in the production of styrene can be categorized as a heterogeneous catalyst, because it is in a different phase than the reactants. The zeolite catalyst is solid and usually supported by an alumina or silica binder to increase its mechanical stability inside the reactor bed. The reactants, on the other hand, are either in the liquid, vapor, or supercritical phase. The production of styrene via alkylation has been done with toluene in the gaseous phase, but it is also possible to use liquid phase alkylation, which requires lower temperatures. Liquid phase alkylation can be more economical in certain situations and can decrease the production of unwanted by-products.
Bulk zeolitic catalysts typically contain an abundance of acid sites. In the presence of alkylation reactions, these acid sites as well as the overall shape selectivity of a typical alkylation zeolite catalyst may contribute to the production of unwanted by-products, such as xylenes.
Therefore, it would be desirable to reduce the amount of the acid sites on a zeolitic catalyst used in the production of styrene. It would also be desirable to use an alkylation catalyst capable of increasing the selectivity to styrene.